1. Field of the Invention
The invention relates to ceramic igniter compositions, and m ore particularly, to such compositions that contain components of a conductive material and insulating mate rial, where the insulating material component includes a relatively high concentration of metal oxide.
2. Background
Ceramic materials have enjoyed great success as igniters in gas fired furnaces, stoves and clothes dryers. Ceramic igniter production requires constructing an electrical circuit through a ceramic component, portion of which is highly resistive and rises in temperature when electrified by a wire lead.
One conventional igniter, the Mini-Igniter(trademark), available from the Norton Igniter Products of Milford, N.H., is designed for 12 volt through 120 volt applications and has a composition comprising aluminum nitride (xe2x80x9cAlNxe2x80x9d), molybdenum disilicide (xe2x80x9cMoSi2xe2x80x9d) and silicon carbide (xe2x80x9cSiCxe2x80x9d). However, while the Mini-Ignite(trademark) is a highly effective product, certain applications require voltages in excess of 120 V.
In particular, in Europe, nominal voltages include 220 V. (e.g. Italy), 230 V (e.g. France), and 240 V (e.g. U.K.). Standard igniter approval tests require operation at a range of from 85 percent to 110 percent of a specified nominal voltage. Thus, for a single igniter to be approved for use throughout Europe, the igniter must be operational from about 187 to 264 V (i.e. 85% of 220 V and 110% of 240 V). Current igniters have difficulty providing such a high and extended voltage range, particularly where a relatively short hot zone length (e.g. about 1.2 inches or less) is employed.
For instance, at higher voltage applications, current igniters may be subject to temperature runaway and thus require a transformer in the control system to step down the voltage. Use of such a transformer device is clearly less desirable. Accordingly, there is a need for relatively small igniters for high voltage applications, particularly over a range of from about 187 to 264 V, which do not require an expensive transformer but still possess the following requirements set by appliance and heating industries to anticipate variation in line voltage:
For a given igniter geometry, one possible route to provide a higher voltage system is by increasing the igniter""s resistance. The resistance of any body is generally governed by the equation
Rs=Ryxc3x97L/A,
wherein
Rs=Resistance;
Ry=Resistivity;
L=the length of the conductor; and
A=the cross-sectional area of the conductor.
Because the single leg length of current ceramic igniters is about 1.2 inches, the leg length can not, be increased significantly without reducing its commercial attractiveness. Similarly, the cross-sectional area of the smaller igniter, between about 0.0010 and 0.0025 square inches, will probably not be decreased for manufacturing reasons.
U.S. Pat. No. 5,405,237 (xe2x80x9cthe Washburn patentxe2x80x9d) discloses compositions suitable for the hot zone of a ceramic igniter comprising (a) between 5 and 50 volume % (xe2x80x9cv/oxe2x80x9d or xe2x80x9cvol%xe2x80x9d) MoSi2, and (b) between 50 and 95 v/o of a material selected from the group consisting of silicon carbide, silicon nitride, aluminum nitride, boron nitride, aluminum oxide, magnesium aluminate, silicon aluminum oxynitride, and mixtures thereof.
Additional highly useful ceramic compositions and systems are disclosed in U.S. Pat. Nos. 5,514,630 and 5,820,789, both to Willkens et al. U.S. Pat. No. 5,514,630 reports that hot zone compositions should not exceed 20 v/o of alumina. U.S. Pat. No. 5,756,215 reports additional sintered compositions that include lead layers that contain up to 2% by weight of silicon carbide.
It thus would be desirable to have new ceramic hot zone igniter compositions. It would be particularly desirable to have new igniter compositions that could reliably operate at high voltages, such as from about 187 to 264 V, especially with a relatively. short hot zone length.
We have now discovered new ceramic compositions that are particularly effective for high voltage use, including over a range of 187 to 264 V.
More specifically, in one aspect of the invention, ceramic hot zone compositions of the invention contain at least three components: 1) conductive material; 2) semiconductor material; and 3) insulating material, where the insulating material component includes a relatively high concentration of metal oxide, such as alumina.
It has been surprisingly found that such high concentration (e.g. at least about 25 or 30 v/o of the insulating material component) of a metal oxide provides a ceramic composition that can reliably provide a high nominal voltage, including 220, 230 and 240 V.
Moreover, ceramic hot zone compositions of the invention have been repeatedly demonstrated to reliably provide a line voltage over an extremely broad, high voltage range, including from about 187 to about 264 V. Hence, igniters of the invention can be employed throughout Europe, and reliably operate within 85 percent and 110 percent of the several distinct high voltages utilized in the various European countries. It also should be appreciated that while certain conventional hot zone compositions may provide a reliable voltage at a specified high voltage, those compositions often fail as voltage is varied over a broader range. Accordingly, the compositions of the invention that provide reliable, prolonged performance over an extended high voltage range clearly represent a significant advance.
While hot zone compositions of the invention are particularly effective for high voltage use, it has been found that the compositions also are highly useful for lower voltage applications, including for 120 V or even lower voltages such as 6, 8, 12 or 24 V applications.
Preferred ceramic igniters of the invention have a hot zone composition comprising:
(a) an electrically insulating material having a resistivity of at least about 1010 ohm-cm;
(b) between about 3 and about 45 v/o of a semiconductive material having a resistivity of between about 1 and about 108 ohm-cm, preferably between about 5 and about 45 v/o of the hot zone composition being composed of the semiconductive material;
(c) a metallic conductor having a resistivity of less than about 10xe2x88x922 ohm-cm,
preferably between about 5 and about 25 v/o of the hot zone composition being composed of the metallic conductor,
and wherein at least about 21 v/o of the hot zone composition comprises a metal oxide insulating material. Preferably, at least about 25 v/o of the hot zone composition comprises a metal oxide insulating material such as alumina, more preferably at least about 30, 40, 50, 60, 70 or 80 of the hot zone composition comprises a metal oxide. insulating material such as alumina, Preferably at least about 25 v/o of the insulating material is composed of a metal oxide such as alumina, more preferably at least about 30, 40, 50, 60, 70, 80 or 90 v/o of the insulating material being composed of a metal oxide such as alumina. Also preferred is where the sole insulating material component is a metal oxide. Preferably the hot zone composition comprises between about 25 and about 80 v/o of the insulating material, more preferably between about 40 and about 70 v/o of the hot zone composition is composed of the insulating material.
Additional preferred ceramic igniters of the invention have a hot zone composition comprising an electrically insulating material having a resistivity of at least about 1010 ohm-cm, with a substantial portion of that insulating material being composed of a metal oxide such as alumina; a semiconductor material that is a carbide such as silicon carbide in an amount of at least about 3, 4, 5 or 10 v/o; and a metallic conductor.
In a further aspect of the invention, preferred ceramic igniters of the invention have a hot zone composition that is substantially free of a carbide such as SiC. Such compositions comprise a metallic conductor and an electrically insulating material having a resistivity of at least about 1010 ohm-cm, with a portion of that insulating material being composed of a metal oxide such as alumina, and the insulating material component also containing a further insulating material that is not an oxide, e.g. a nitride such as AlN. Such compositions may contain the same or similar amounts as discussed above for the tertiary insulating material/semiconductor material/electrically conducting material compositions.
Hot surface ceramic igniters of the invention can be produced with quite small hot zone lengths, e.g. about 1.5 inches or less, or even about 1.3, 1.2 or 1.0 inches or less, and reliably used at high voltages, including from about 187 to 264 V, in the absence of any type of electronic control device to meter power to the igniter. It will be understood herein that for multiple-leg geometry ingiters (e.g. a hairpin slotted deign), the hot zone length is the length of the hot zone along a single leg of the multiple-leg igniter.
Moreover, igniters of the invention can heat rapidly to operational temperatures, e.g. to about 1300xc2x0 C., 1400xc2x0 C. or 1500xc2x0 C. in about 5 or 4 seconds or less, or even 3, 2, 5 or 2 seconds or less.
Preferred hot zone compositions of the invention also can exhibit dramatic high temperature capability, i.e. repeated exposure to high temperatures without failure. The invention thus includes ignition methods that do not require renewed heating of the igniter element with each fuel ignition. Rather, the igniter can be continuously run at an elevated ignition temperature for extended periods to provide immediate ignition e.g. during a flame-out. More specifically, igniters of the invention can be run at an elevated temperature (e.g. about 800xc2x0 C., 1000xc2x0 C., 1100xc2x0 C., 1200xc2x0 C., 1300xc2x0 C., 1350xc2x0 C. etc.) for extended periods without a cooling period, e.g. at such temperatures for at least 2, 5, 10, 20, 30, 60, 120 minutes or more.
Other aspects of the invention are disclosed infra.